A clamp-on type ultrasonic flowmeter for installing a detector on the outer wall of a pipe, emitting an ultrasonic wave into a fluid flowing in the pipe from the outside of the pipe, and measuring a flow rate on the inside of the pipe by measuring a change of the ultrasonic wave propagating within the fluid has many advantages such as an existing pipe not requiring specific installation work, and a minimal influence by the temperature or pressure of the fluid or its corrosiveness.
There are known techniques as a flow rate measurement method for such a flowmeter, such as the pulse Doppler method and the transit time method.
A flow rate measurement by the pulse Doppler method has at least one detector with an integrated transmitter-receiver emitting an ultrasonic pulse into a fluid as the subject of measurement and receives an ultrasonic echo wave reflected by a foreign body such as a bubble mixed in the fluid as shown by FIG. 1A.
This is an application of the principle that the frequency of the echo wave shifts by an amount in proportion to a flow velocity. Since the echo wave returns quickly from a part of a fluid close to the detector, and the return time is delayed with distance, the use of the phenomenon obtains a flow velocity profile Vx at positions along the traverse line and then an integration of the distribution across the whole section (A) of the pipe obtains a flow rate as expressed by (1).
[Expression 1]Q=∫Vx·dA   (1)
This method is capable of a high precision and high speed response, and has excellent anti-bubble qualities. However, the method is faced with a technical problem of incapability of measuring a fluid with a small amount of impurities and of a limitation of a measurable velocity range.
A patent document 1 has noted the measurable velocity range. That is, the maximum measurable velocity VMAX is expressed by:
[Expression 2]VMAX≦Cf2/(8·D·f0·sin θf)   (2);
where Cf is the sonic velocity of a fluid, D is the inner diameter of the pipe, and f0 is the transmission frequency of an ultrasonic wave.
This is because the pulse Doppler method figures out fd by sampling a Doppler shift frequency fd at a repetitive frequency fprf as shown by FIGS. 1B and 1C, and accordingly, it is necessary that:
[Expression 3]Vprf≧2·fd   (3),
according to the sampling theorem. Meanwhile, in order to measure a flow velocity profile over the entire area of a pipe along the measurement line, because it is not possible to carry out a subsequent measurement until the return of echo waves from the pipe wall on the other side of the pipe, it is necessary that:
[Expression 4]Vprf≦Cf/(2·D)   (4)
Furthermore, when the velocity of a fluid under measurement is Vf, the Doppler shift frequency fd is expressed by:
[Expression 5]fd=2·Vfsin θf·f0/Cf   (5)
A combination of the expressions (3) through (5) results in the expression (2), making it apparent that there is an upper limit to the measurable flow velocity.
Another problem with regard to the pulse Doppler method is the fact that it is not possible to detect the flow velocity close to the pipe wall on the detector side. That is, a flow rate measurement by the pulse Doppler method is capable of measuring a flow velocity profile if at least a detector with an integrated transmitter/receiver is used, but the velocity measurement accuracy is degraded close to the pipe wall on the detector side. As a counter measure to the problem, a patent document 2 has disclosed a method for acquiring a flow rate of a fluid by extrapolating the normally detected flow velocity of a pipe wall part on the opposite side to the pipe wall part equipped with the detector. And a patent document 3 has disclosed a method for making two divided distributions, by dividing a measured velocity distribution into two at the center of the flowing fluid section and acquiring a flow velocity of the entire flowing fluid section by folding one of the divided distributions with a smaller fluctuation.
Both these methods, however, assume the flow of a fluid to be a convex and symmetrical flow and result in degraded flow rate measurement accuracy for asymmetrical flows such as a flow at a bend or at a merge. Also assumed is that the flow only has an axial component, thus degraded flow rate measurement accuracy results if a radial component occurs in a flow at a bend or at a merge.
On the other hand, the transit time method is a method which employs a pair of detectors integrated with transmitter/receiver as shown by FIG. 2A, and compares an ultrasonic transmission time T1 (refer to FIG. 2B) from the upstream to downstream side with an ultrasonic transmission time T2 (refer to FIG. 2C) from the downstream to upstream side and acquires the average flow velocity V and flow rate Q according to the expressions (6) and (7).
                    [                  Expression          ⁢                                          ⁢          6                ]                                                                      V          t                =                              D                          sin              ⁢                                                          ⁢              2              ⁢                                                          ⁢                              θ                f                                              ⁢                                    Δ              ⁢                                                          ⁢              T                                                      (                                                      T                    o                                    -                  τ                                )                            2                                                          (        6        )                                [                  Expression          ⁢                                          ⁢          7                ]                                                                      Q          =                                    π              4                        ⁢                                          D                2                            ·                              1                K                            ·                              V                t                                                    ;                            (        7        )            
where ΔT=T2−T1; D: pipe diameter; θf: angle of incidence of ultrasonic wave into a fluid; T0: a propagation time (=(T1+T2)/2) in still water; τ: a propagation time in a pipe wall and wedge; K: a conversion coefficient for the average flow velocity.
While the method has problems, such as a low accuracy, a slow response and a vulnerability to bubbles or impurities, as compared to the above described pulse Doppler method, it has advantages such as the capability of measurement of a fluid without bubbles or impurities, and an absence of a limitation of a measurable range contrary to the pulse Doppler method.
As described so far, there are advantages and disadvantages to both the pulse Doppler method and the transit time method, since the conventional method for measuring a flow rate using a single measurement instrument utilized either the pulse Doppler method or the transit time method, is faced with the technical problem of a reduced measurement accuracy or inability of measurement depending on the velocity of a fluid as the subject of measurement or the conditions such as inclusion of bubbles.
[Patent document 1] laid-open Japanese patent application publication No. 2004-12205
[Patent document 2] laid-open Japanese patent application publication No. 10-281832
[Patent document 3] laid-open Japanese patent application publication No. 2004-12204